> purchase a burner laptop with cash

Easy.

> and not in sight of any cameras.

Almost impossible, unless you're buying from a fence. Laptops are high-value items; almost every place that sells them has cameras on the area where they are sold.

> Is this sufficient operational security to remain totally anonymous against every reasonable threat save for an extremely motivated nation state?

You go to those extremes, and you're likely to motivate a nation state...